H-phosphonate method wherein hydroxyl group-selective condensation is carried out by means of a phosphonium-type condensing agent, BOMP, has been known for the method of synthesizing DNA without protecting a base moiety (Non-Patent Document 1). This reaction utilizes the phenomenon that an active phosphite intermediate generated during the condensation reaction will react more preferentially with a hydroxyl group than with an amino group in the base moiety.
[Non-Patent Document 1] Wada, T.; Sato, Y.; Honda, F.; Kawahara, S.; Sekine, M., Journal of the American Chemical Society 1997, 119, 12710-12821
[Non-Patent Document 2] Gryaznov, S. M.; Letsinger, R. L., Journal of the American Chemical Society 1991, 113, 5876-5877